1. Field of the Invention
The present invention relates to a fuel feed pump for feeding high pressure fuel to fuel injectors used for in-cylinder fuel injection system. More particularly, the invention relates to a fuel feed pump having an improved structure for a piston (plunger) that is driven by a cam linked to an engine.
2. Prior Art
The fuel feed pump that feeds high pressure fuel to fuel injectors for direct injection system has heretofore employed a piston structure that is driven in the axial direction by contacting with a cam that is driven or rotated by the output of an engine.
In this type of fuel feed pump, the durability of the piston structure having a portion that slides on a cam, hereafter termed a sliding portion must be maintained. In the fuel feed pump disclosed in, for example, Japanese Patent App. Laid-open No. 10-82354, wear resistance is maintained by attaching a separate shoe to the end surface of the plunger that slides on the outer peripheral surface of the cam.
FIG. 2 is a sectional side view illustrating an essential portion of a conventional fuel feed pump, and shows a structure having a sliding portion which is separate from a plunger at the end of the plunger.
In FIG. 2, the fuel feed pump 1 has a plunger 3 in a casing 2 that serves as the upper main body, the plunger 3 being accommodated so as to be slidable in the vertical axial direction in FIG. 2.
A tappet 4 which is separate from the plunger 3 is integrally secured to an end of the plunger 3. The tappet 4 is made of a material such as a very hard metal having excellent wear resistance. A cam 5 which is rotated by the output of the engine abuts the end surface of the tappet 4.
The cam is linked to the drive shaft of the engine (not shown) and has multiple lobes 5a at equal intervals on the outer peripheral surface thereof in order to reciprocatingly drive the plunger 3 via the tappet 4.
The casing 2 includes a compression spring 6 for urging the plunger 3 in a direction in which it comes into pressing contact with the cam 5, a cylindrical sleeve 7 for guiding the lateral surfaces of the plunger 3 in the axial direction, and a housing 8 for accommodating the sleeve 7.
Further, a bracket 9 is integrally attached to the casing 2 to contain the housing 8 and the sleeve 7.
The end surface of the tappet 4 protrudes through an opening 9a of the bracket 9.
A holding member 10 in the shape of a flange is press fitted on to the periphery of the end portion of the housing 8, and an oil seal 11 made of rubber is slidably brought into contact with the plunger 3.
A heat insulator 12 is provided on the engine mount portion on the periphery of the bracket 9 to suppress the conduction of heat from the engine side (not shown) to the pump body (casing 2).
A pump chamber 13 is formed at an upper end of the plunger 3, and is communicated with a fuel (gasoline) introduction passage 14 and a delivery passage 15.
The introduction passage 14 and the delivery passage 15 are communicated with an introduction port and a delivery port (not shown) formed in the casing 2.
The introduction passage 14 and the delivery passage 15 are provided with a valve structure for the pump chamber 13, in order to introduce and deliver fuel according to the reciprocating motion of the plunger 3.
The introduction port in the casing 2 receives fuel fed from a low-pressure pump (not shown), and the delivery port delivers the pressurized fuel to the injector (not shown) through a high-pressure conduit.
Ring-like seals are interposed at the joint surfaces of each member in order to prevent fuel leakage.
In the fuel feed pump 1 shown in FIG. 2, the plunger 3 and the tappet 4 are reciprocatingly driven by the cam 5 that moves or rotates in synchronism with a control stroke of each cylinder of the engine and opens and closes the introduction passage 14 and the delivery passage 15 to supply high pressure fuel into the injector.
In order to improve the resistance of the sleeve 7 against seizure (resistance against adhesion), the surface of the plunger 3 is made of, for example, a metal material which is heat-treated and then coated with CrN or NiP.
The plunger 3, therefore, is formed with a high degree of hardness on the surface of the sliding portion corresponding to the sleeve 7, and with small surface roughness (improved smoothness).
Further, the tappet 4 that slides with the cam 5 is machined separately from the plunger 3 that slides relative to the sleeve 7, and a surface treatment is performed on the portion that slides (sliding portion) on the cams so that it will not be damaged. The tappet 4 is assembled so as to be integral with the plunger 3 and is mounted in the casing 2.
According to the conventional fuel feed pump 1 described above, the piston portion that slides on the cam 5 has a tappet 4 which is separate from the plunger 3. Therefore, the plunger 3 and the tappet 4 must be separately machined and assembled. Moreover, the surfaces of the plunger 3 and the tappet 4 must be polished and coated, thus creating a problem in that with the increased number machining steps the costs cannot be decreased.